Subscribe to our Newsletter

Truck maintenance is one area where knowing component fundamentals can allow you to better spec vehicles to match your applications, or spot trouble before it blossoms into a catastrophic failure.

To test a transmission properly, you need to climb behind the wheel
Aside from the engine, no systems affect a truck’s performance more than the transmission, its suspension, shock absorbers and steering system. And while nothing beats having a trained mechanic look over a vehicle, drivers can report in-cab feedback and perform simple infield checks to prevent minor problems from developing into costly or even life-threatening ones. Taking a close look at the following factors will help you spec and check your heavy-duty trucks.

Transmissions are complex mechanisms and endure a great deal of abuse in vocational applications. Fortunately, if they are properly cared for, they can last many years – even in extreme operating conditions. As with any mechanical component, the key to avoiding catastrophic transmission failures is early detection. The problem is that many clues to transmission failure manifest themselves over time. If you have the same driver behind the wheel of the same truck day in and day out, there’s a good chance a transmission’s deterioration will be so gradual the driver won’t notice something’s wrong until a failure is eminent.

With manual and automated manual transmissions, most problems are clutch related. A loose or slack pedal is a sure sign of excessive wear. An easy way to check for a worn clutch is to start a truck’s engine and shift the transmission into neutral. With the engine idling, fully depress and release the clutch while listening for any grinding or grating noises. The noises are most apparent when the pedal is all the way up or down, so be sure to hold those positions for a moment when listening.

When driving a truck with a manual transmission, all shift points should be smooth without any accompanying noises (usually whining or humming sounds). Watch for any catch points as you shift gears, or jerking when the clutch is released. One good way to check for clutch slippage is to suddenly gas the truck while cruising in second or third gear. If the engine rpms increase without a corresponding increase in vehicle speed, you’re going to have to replace the clutch.

Since automatic transmissions are hydraulically actuated, checking the state of the transmission fluid for leaks is the logical place to begin a troubleshooting session. Obviously the transmission should be operated with the proper level and type of fluid in it. Many automatic transmission problems such as slow gear engagement and gear slippage can be corrected by simply filling the transmission fluid to the proper operating level.

The fluid itself should be clean. Dark fluid with a noticeably burnt smell should be changed as soon as possible. It’s also a good idea to wipe a transmission dipstick on a dark shop rag and look for the glint of metal shards or fragments present in the fluid. If you see metal in the fluid, that’s a sure sign of grinding gears and the need for immediate attention.

Test-driving an automatic transmission can also give you idea of whether service work will be needed in the immediate future. First, make sure the transmission is at its normal operating temperature before evaluating it. While still parked, shift the transmission through its gears and note the length of time it takes for each gear to engage. With each shift, the gear should engage smoothly and quickly. The truck should feel like it wants to creep forward and there should be no “clunking” noise or jerking motion as the gears mesh. Be alert for any long delays after shifting into “drive” or “reverse” before the transmission kicks in. Out on the road in “drive” the transmission should again smoothly shift through its gears with little or no jerking or slippage.

Worn shocks can cost a truck up to 10 feet in stopping distance
Shock absorbers are perhaps the least glamorous component on a vocational truck, but the job they do is essential to productivity and long-term vehicle life. Shocks absorb vibration energy generated as the truck moves down the road. It then coverts that energy into heat and dissipates the heat away from the vehicle. A shock’s dampening forces are generated hydraulically by the flow of oil through one or more valves. The flow of the oil through these valves is governed by a system of springs and orifices. Shocks are speed-sensitive. The faster they move, the more resistance they create to counter the higher vibration energy they receive.

A properly tuned shock absorber enhances tire life and overall safety by minimizing wheel hop. In fact, a heavy-duty truck fitted with worn-out shocks can lose as much as 10 feet of overall stopping distance when the brakes are applied due to wheel hop. At the same time, properly tuned shocks greatly enhance vehicle control and stability while insuring better driver performance and productivity by delivering a comfortable ride.

To properly assess the condition of a truck’s shock absorbers you’re going to have to get down on the ground and crawl underneath the truck. (You should, of course, follow all normal safety procedures before venturing underneath any vehicle, including chocking the wheels, setting the parking brake and ensuring the engine is off and the transmission is locked in first gear for manual transmissions or “park” for automatic units.)

Each shock needs to be checked individually. The first thing you need to look for is integrity at the unit’s connection points to the truck. Check for signs of worn bushings, broken mounts and – on the shock itself – broken dust tubes (the shock’s exterior body).

Next, look for obvious signs of leaking oil. Usually, these will show up as oil streaks, drips or drops accumulated on the bottom of the shock assembly. Remember though, that all shock absorbers emit small amounts of oil in the normal course of their lives. This process, called “misting,” is completely normal. As a rule of thumb, remember that a misting shock will look somewhat oily and dusty. A leaking shock will be wet to the touch.

To check if a shock is functioning properly, run the truck up the road (a few miles should suffice) and then place your hand on the lower part of the shock tube once you’ve returned. If the shock absorber feels noticeably warmer than the surrounding frame, then it is performing properly.

Free play in the steering wheel? Not a good sign.
A truck’s linkage and suspension system directly affect steering performance. Poor front-end alignment, worn ball joints, bushings and damaged tie rods can all adversely affect a truck’s steering characteristics. Again, a truck’s tires must be properly inflated and balanced, while the hydraulic steering pump should be operating at the correct pressure and flow rates. If not, steering response and vehicle control will suffer.

A leaking steering pump or loose drive are obvious problems that can be detected with quick, under-the-hood inspections. Other steering problems will manifest themselves on the road. To that end, truck drivers should always report any free play or lash in the steering wheel. A truck shouldn’t wander all over the road. And if a steering system is tight, it won’t. The steering wheel’s resistance (or the effort it takes to turn the wheel) should be consistent from one steering stop to the other. There should be no catches as the steering wheel rotates, nor should there be any noise from either the steering pump or the linkage itself.

Front end hopping around? Then tie rod failure could be imminent
One final system you can easily troubleshoot on your work trucks is the suspension. On the front end of a truck, tie rod failure is commonplace enough to warrant driver vigilance for any potential problems.

Tie rods are critical, adjustable links used to connect left and right front wheel assemblies and coordinate accurate steering characteristics. Most tie failures begin as maladjustments and progress to full-blown failures. In many cases, a tie rod progressing toward a catastrophic failure will reveal itself early with uneven front tire wear or premature wear of other steering and front suspension components.

Loose or erratic steering is another danger sign, as is truck instability or unusual rattling noises or vibrations. If your drivers report difficulty maintaining even tire contact with the road, an immediate front end check is warranted. Remember that in worse cases, tie rod failures can lead to complete loss of vehicle control.

Regardless of the rear suspension type spec’d, most vocational trucks are equipped with sliding-bearing, or split-bearing, torque rods. These rear suspension support rods help maintain an axle’s position under a truck’s frame. They also flex and transmit acceleration, braking and cornering forces between the frame and the axle. Sliding- or split-bearing torque rods provide the high degree of articulation required in off-highway applications.

Like a tie rod, torque rod failures can lead to loss of vehicle control. A potential torque rod failure shows itself in many different ways. Truck wandering is commonly reported as major indicator. Unlike tide rod failure, the driver will feel as if the rear body of the truck is attempting to wander, instead of the front end.

As with many suspension and steering system problems, excessive tire wear is once again a reliable indicator that a problem exists. Other warning signs include U-joint failures, axle housing fatigue or failures, ring gear and pinion failures and other driveline-related problems.
—Information provided by ZF North America.